1. Field of the Invention
The present invention relates to imaging systems, and more specifically to a mobile imaging system for medical diagnosis.
2. Background of the Invention
A typical positron emission tomography (or PET) scan is a non-invasive imaging process that uses a radioactive tracer or radiopharmaceutical to create 3-dimensional, color images of the functional processes of certain tissues and organs within the human body. PET scan technology is widely used in oncology to assist with detecting and staging certain types of cancer including breast cancer, lymphoma, and certain types of lung cancer. PET scanning technology is also widely used to diagnosis and assist in determining treatment options for a variety of brain-related or nervous system-related disorders and diseases, such as epilepsy and Alzheimer's disease. PET scans can also provide important information regarding the functioning of the brain. Heart disease, heart-related damage or scarring, and the general working of the heart are detectable using PET scan technology.
In order to prepare for a PET scan, commonly a patient first ingests, inhales, or is injected with a radiotracer or radiopharmaceutical. The radiotracer or radiopharmaceutical can be specific for the tissue or organ of interest. A radiotracer is a radioactive isotope that has been tagged or attached to a natural chemical. This natural chemical can be, for example, glucose, water, or ammonia. Once introduced, the radiotracer circulates throughout the body and becomes more concentrated in tissues that utilize the natural chemical. For example, the radioactive drug fluorodeoxyglucose (FDG) is commonly tagged to glucose. The glucose then concentrates in those pasts of the body that use glucose for energy. For example, cancer often uses glucose in greater amounts than normal tissue, causing FDG to become more concentrated in cancerous tissues.
As the radioisotope undergoes positron emission decay within the body, it emits a positron, also known as an “antiparticle” of the electron, but with opposite charge. As the positrons move through the body they encounter electrons. These encounters annihilate both the positron and the electron, resulting in the creation of a pair of photons moving in opposite directions. Photons are detected after they exit the body and reach a scintillator of luminescent material or a detector which converts positron energy into an electric signal. The interaction of the photons with the scintillator creates a burst of light or an “event” at is detected and enhanced by an array of photomultiplier tubes, silicon avalanche photodiodes (Si APD), or other similar devices. When two oppositely disposed gamma photons each strike an oppositely disposed photomultiplier tube they produce a time coincidence event. The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted and moving apart at almost 180 degrees. Sophisticated computer and software technology makes it possible to localize their source along a straight line of coincidence or the “line of response” (LOR). In practice the LOR has a finite width as the emitted photons are not exactly 180 degrees apart.
Time-of-flight (TOF) refers to the difference between the detection times of the two coincidence events arising from a single positron annihilation event. TOF measurement allows the annihilation event to be localized along the LOR with a resolution of about 75-120 mm FWHM (full width, half maximum), assuming a time resolution of 500-800 ps (picoseconds). Though less accurate than the spatial resolution of the scanner, this approximate localization is effective in reducing the random coincidence rate and in improving both the stability of the reconstruction and the signal-to-noise ratio (SNR), especially when imaging large objects. After being sorted into parallel projections, the LOR defined by the coincidence events are used to reconstruct a three-dimensional distribution of the positron-emitting radiotracer within the patient.
PET scan images can be combined with computed tomography (CT) images or magnetic resonance imaging (MRI) in a process called co-registration or image fusion. PET scans are often combined with CT scans to provide detailed anatomical and functional information about the organs and tissues. Results from PET/CT scans typically give additional diagnostic and treatment information than PET scan results alone. PET and PET/CT scans are most commonly used to help diagnose and re-stage cancer, evaluate the heart muscle, and detect brain abnormalities. In all cases, interpreting PET/CT or PET scan results can be a very complicated process and more generally related to oncological applications and generally be served for physicians and radiologists who have received specialized training in nuclear medicine and CT diagnosis.
PET scans differ from other imaging tests, such as Magnetic Resonance Imaging (MRI) and CT scanning, by the ability to detect changes in the body at the cellular level. The images obtained by PET scan can reveal how a tissue or organ is functioning, rather than just how it looks. This allows for the detection of a disease much earlier, often before it has progressed enough to actually affect the surrounding tissue or organs.
Despite the advantages, there are limitations to the widespread se of PET. One reason is the high cost of cyclotrons needed to produce the short-lived radionuclides for PET scanning. On-site chemical synthesis apparatus are usually required to produce the necessary radiotracers used in the process. Hospitals and universities are often incapable or unwilling to maintain such systems. As a result, third-party suppliers provide radiotracers and often supply many sites simultaneously. This limitation restricts clinical PET primarily to the use of tracers labeled with Fluorine-18, which has a half life of 110 minutes and can be transported a reasonable distance before use. Rubidium-82, which can be created in a portable generator, provides ready accessibility and easy ease of use and may be used for myocardial perfusion studies.
A further disadvantage may be the size of PET scanners themselves and ease of use, which are typically large, bulky machines that must be installed in a single location. Because of their energy and power requirements, they are usually hardwired into the building electrical system. This immovability necessitates the transport of patients to the machine, which can be difficult or impossible, in certain situations. This limits their use to those patients in the immediate vicinity or to those that are, or can be made, mobile.
Cardiac diseases are widespread in US. PET scanners are very effectively used in determining the effects of a heart attack, or myocardial infarction, on areas of the heart. They are also useful at detecting blood flow, or loss thereof, to the heart muscle and identifying areas of the heart muscle that would benefit from a procedure such as angioplasty or coronary artery bypass surgery (typically in combination with a myocardial perfusion scan). The cost of using PET scanners is not only limited by the capital cost of acquiring PET scanners but also investment adapting facility to house the PET scanners and all ancillary equipment. The cost and limitations of PET make it unlikely that a large number of physicians offices or a larger number of hospitals will be able to acquire such devices.
There is currently a need for a portable PET scanner. There is a more pressing need for a portable PET scanner capable of providing wide access to variable clinical information. A portable PET scanner could allow multiple physicians to share the costs and benefits, providing more convenient and immediate access to healthcare professionals and patients.